Granulocyte colony-stimulating factor (G-CSF) is the standard growth factor for mobilizing hematopoietic progenitor cells in healthy peripheral blood progenitor cell (PBPC) donors. G-CSF is given daily for 5 days to PBPC donors prior collecting PBPC concentrates by apheresis. PBPC donors given G-CSF experience splenic enlargement and, rarely, spontaneous rupture of the spleen. The magnitude of spleen length increase is highly varialbe among individuals and is not affected by donor gender, race, or age. We are currently investigating the mechanisms of G-CSF mobilization of stem cells into the peripheral blood. Plasma protein profiles are being compared before and after G-CSF administration. Associations between the expression of cell adhesion molecules on mature neutrophils and neutrophil precursors and spleen size change will also be investigated.[unreadable] [unreadable] An agent that mobilizes stem cells by a different mechanism than G-CSF is also being used clinically to mobilize stem cells. This agent, AMD3100, mobilizes stem cells within a few hours after it is given by directly disrupting the binding of CXCR4 on stem cells with SDF-1 or CXCL12 on marrow storma. In constract, G-CSF mobilizes stem cells after it has been given for 4 to 5 days by the release of proteolytic enzymes that degrade adhesion molecule and by the down-regulation of expression of adhesion molecules. Prelimenary data suggests that stem cells mobilized by AMD3100 and G-CSF have different properties and functions. We have begun to investigate differences in hematopoietic stem cells mobilized by AMD3100 and G-CSF using different gene expression microarrays and micro RNA expression microarrays.